This invention relates to a fish culturing method and a sac-fry pond for use in practicing the method and more particularly, to a fish culturing method capable of rearing lively sac-fry from an eyed or hatching stage to a release or adult stage and controlling a sac-fry growth rate, and also to a sac-fry pond provided with a fish culturing means.
As the rearing of fry has been popularized so as to secure the fishery resources, problems have arisen as known well, which include a difficulty in securing water of a suitable temperature. In a district having a river to which for example salmons and trouts are expected to return, an artificial propagation of fish and fish culturing activities have been extensively carried out. As the districts in which the artificial propagation of fish and the rearing of fry are done increase, it becomes difficult to secure water of a preferable quality which has an optimum fry-rearing temperature (approximately 8.degree. C). The temperature of spring water in Hokkaido, Japan, is approximately 8.degree. C. but a sufficient quantity of such spring water can be obtained in limited districts only. Since it is difficult to obtain a required quantity of such spring water, water from some other sources of water supply, for example, river water and river-bed water have come to be used today.
Salmons and trouts spawn in autumn, and juveniles obtained from the spawn are released in the following year, so that it is necessary to collect water in a cold season. The artificial propagation of salmons and trouts is carried out extensively in Hokkaido and Tohoku districts in Japan. In some areas of these districts in which spring water cannot easily be obtained, river-bed water is generally used. The temperature of the river-bed water in these districts in the fish culturing season, i.e., in a period of time of from autumn to spring in the following year is as low as 1.degree. to 4.degree. C. Consequently, rearing of fry is hindered in these fish culturing places.
The steps of growth of salmons and trouts from the collection of spawns to the swimming-up of fry are expressed on the basis of the daily cumulative water temperature obtained by multiplying the number of fry-rearing days counted from the day of fertilization by an average water temperature (.degree. C.) of the same days. According to this system of expressing the standard steps of growth of salmons and trouts based on the daily cumulative water temperature, substantially 450.degree. to 710.degree. C. corresponds to the step from the fertilization to hatching, and substantially 960.degree. to 1100.degree. C. to the step from the fertilization to swimming-up. Feeding is started when the fry begin to swim up, and such fry are reared in a sac-fry pond of a larger depth until the fry have grown up to such an extent that they can be released.
Therefore, if salmons and trouts are reared at a suitable water temperature of 8.degree. C. and a lower water temperature of, for example, 4.degree. C. throughout the fish culturing period from the collection of spawns to the swim-up of fry, the fry-rearing period counted from the day of collection of spawns in the former case is substantially 125 days (1000.div.8), while the fry-rearing period counted from the day of collection of spawns in the latter case is 250 days, i.e., a very large number of days are required. The results of researches show that, at a water temperature other than a suitable water temperature, the fry grow more slowly than at a suitable water temperature. Moreover, the estimated return rate of the juveniles reared at a suitable water temperature and released is about 3%, whereas the estimated return rate of the juveniles reared at a lower temperature and released is not more than 1.5% which is far lower than the above percentage.
In order to eliminate these inconveniences, it becomes necessary that the fish culturing water be heated so as to maintain the temperature of the water at a suitable level. Since it is necessary that the fry of salmon be reared in flowing water, a great deal of quantity of heat is required. This causes a great increase in the cost of maintaining the temperature of the water in use at a suitable level. Another problem resides in that heating the water collected from a river causes a decrease in the content of the dissolved oxygen, and the derangement of the ecosystem of the microorganisms in the collected water. Therefore, heating such water is not preferable.
Rearing fry at a low water temperature has another problem. Namely, rearing fry at a low water temperature causes not only a delay in the growth of the fry but also an increase in the death rate of the fry and a decrease in the yield of the fish. The fry have a habit of swimming against a flow of water seeking for more agreeable environment, and, accordingly, they gather at a position in the vicinity of a water feed port of a water tank. In the natural environment, salmons and trouts grow as they stay still at the same positions from the stage of hatching to that of swim-up. Therefore, it is not preferable in view of fry-rearing management that the fry gather at one position, and rearing fry as they are left gathering at one position has great influence upon the fish-culturing results. Consequently, it is important to develop fry-rearing techniques for making environment in which the fry grow as they stay still at the same positions from the stage of hatching to that of swim-up.